This invention relates to a method of strengthening of a bolt hole and more particularly, but not by way of limitation, to a method of strengthening the bolt hole in a fibrous composite laminate.
Heretofore, in the design of fibrous composite parts, ply padups have been required when holes are drilled in laminate sheets. This extra material is required because of reduced allowables due to notch sensitivity of the laminate caused by interlaminar shear stresses around the edges of the bolt holes. These interlaminar shear stresses are a result of edge effects and delamination caused by the relatively low material properties of the "through-the-laminate" direction.
Current methods of suppression of edge effects around bolt holes are through the use of stitching of a B-staged laminate and providing quasi-isotropic padups in the area of the bolt holes. A limitation to this stitching method is that the laminate can only be stitched while B-staged. Once cured, the laminate cannot be altered. Providing quasi-isotropic padups in areas to be bolted works fairly well in increasing laminate bearing strength, but the reduced longitudinal stiffness requires a substantial increase in laminate thickness which is directly related to increased weight.
Further, highly unidirectional composite laminate sheets exhibit poor bolt hole properties such as thickness strength, bearing strength and galvanic corrosion associated with conventional aircraft fasteners.
In the following U.S. Pat. Nos. 2,700,172 to Rohe, 3,158,503 to Young, 3,526,072 to Campbell, 3,895,409 to Kwatonowski, 3,977,146 to Wiley, 4,098,922 to Dinella et al, 4,118,855 to Lequeux, 4,232,496 to Warkentin and 4,296,586 to Heurteux various types of fastening devices and grommet assemblies are shown which are used with different types of airframe structures and composite materials. None of these prior art patents point out the distinguishing features of the subject method of bolt hole strengthening as described herein.